Vaccine for protection against shigella sonnei disease

ABSTRACT

Compositions and methods for protecting a susceptable host against an infection of  Shigella sonnei  are disclosed. Such compositions and methods are useful for protecting the host against bacillary dysentery and shigellosis.

RELATED APPLICATION DATA

The present application is a divisional of U.S. patent application Ser.No. 12/474,223, filed May 28, 2009; which is a continuation of U.S.patent application Ser. No. 10/346,706, filed Jan. 15, 2003, now U.S.Pat. No. 7,541,043, issued Jun. 2, 2009; which is a nonprovisional ofU.S. Provisional Patent Application No. 60/349,788, filed Jan. 16, 2002;all of the foregoing applications are incorporated herein by referencein their entireties.

STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT

This invention is owned by the United States Government.

SEQUENCE LISTING

The Sequence Listing text file attached hereto, created Nov. 14, 2001,size 194 kilobytes, and filed herewith as file name“6137FDA1CON1_SEQ_(—)20111114_ST25.txt” is incorporated herein byreference in its entirety.

TECHNICAL FIELD

This invention relates to the field of vaccines for treating andpreventing bacillary dysentery. In particular, this invention providesfor attenuated live bacteria expressing the Shigella sonnei form I-Opolysaccharide that are useful for inducing an immunoprotective responseagainst Shigella sonnei.

BACKGROUND

Bacillary dysentery and specifically shigellosis is a global humanhealth problem. It has been over 100 years since the discovery ofShiga's bacillus, yet shigellosis remains endemic in most areas of theworld including industrialized nations. An estimated 200 million peopleworldwide suffer from shigellosis, with more than 650,000 associateddeaths annually (27). A recent CDC estimate indicates the occurrence ofover 440,000 20 annual shigellosis cases in the United States alone(32), approximately 80% of which are caused by Shigella sonnei. Allvirulent S. sonnei strains comprise a single serotype determined by formI O-polysaccharide (O-Ps). This O-Ps is composed of a disacchariderepeating unit containing two unusual amino sugars,2-amino-2-deoxy-L-altruronic acid (L-AltNAcA) and2-acetamido-4-amino-2,4, 6-trideoxy-D-galactose (4-n-D-FucNAc) (25). The25 genes encoding the enzymes that produce this O-Ps are novelly locatedon the 180 kb virulence plasmid in S. sonnei (26), which also harborsthe invasion genes (36). Virulent form I colonies are typically unstableand upon replating convert to rough colonies, termed form II, dueprimarily to spontaneous loss of the large virulence plasmid and theensuing loss of form I O-antigen. Substantially identical genes thatencode the same antigen producing enzymes are located on the bacterialchromosome in Plesiomonas shigelloides (termed the O17 gene cluster).

Immunity to Shigellae, acquired either by natural infection or volunteerchallenge, is mediated largely by immune responses directed against theserotype specific O-Ps (9, 10). This insight has led to the developmentof a variety of candidate vaccines containing Shigella O-Ps for oral orparenteral administration including recombinant heterologous, live,bacterial carrier strains (3, 12, 18). Parenteral vaccines in the pasthave not been effective in protecting against bacillary dysenterybecause shigellosis is an infection limited to the superficial layer ofthe colonic mucosa. It is, therefore, not surprising that attempts toimmunize man or other primates with killed whole cell Shigella vaccines,administered by the parenteral route, have not been successful.

In early recombinant vaccine efforts, the virulence plasmid of S. sonneiwas transferred as part of a larger plasmid cointegrate to theattenuated vector Salmonella enterica serovar Typhi strain Ty21a (i.e.S. Typhi Ty21a) (12). The resulting hybrid vaccine strain, 5076-1C,expressed S. sonnei O antigen as a lipid-linked surface O-Ps as well asS. Typhi 9,12 LPS (37). Although not core-linked, this form I O-Ps wasimmunogenic (12) and oral immunization of volunteers with 5076-1Celicited protection against virulent S. sonnei oral challenge (3, 21,40). However, the protection observed in volunteers was variable,presumably due to loss of the form I gene region from the largecointegrate plasmid in 5076-1C (17). Thus, further molecular studies areneeded to stabilize the S. sonnei form I gene region in vaccine vectorconstructs. In spite of an increased molecular understanding of Shigellapathogenesis, there are still no licensed vaccines for protectionagainst shigellosis in the United States.

Although the form I O-Ps-encoding locus has been studied in some detailpreviously (6, 24, 38, 42, 45) the biosynthetic pathway and minimal generegion for stable expression of O-antigen have not been unambiguouslydefined. We show through deletion and sequence analyses and LPSexpression studies that the S. sonnei form I biosynthetic gene regioncomprises a 12.3 kb operon. A detailed biosynthetic pathway, based onDNA sequence analysis of this region and the known structure of form IO-Ps, is proposed. In addition, stable expression of form I O-Ps wasobserved from a low copy plasmid and was associated with the removal ofan adjacent 1591 resulting in small, genetically stable form I generegion constructs. We report the development and animal testing of alive attenuated S. Typhi vaccine vector stably expressing enzymes thatproduce form I O-Ps for protection against S. sonnei disease.

To develop a more stable living attenuated oral Shigella strain vaccine,the gene region encoding the enzymes that produce form I antigen wasisolated from a large non-conjugative plasmid and analyzed to determinethe essential genes required for biosynthesis of Shigella sonnei form IO-polysaccharide. Nucleic acids totaling 18 kb, were characterizedgenetically and used to define a minimal region encoding all of theproteins required to produce the form I antigen for development of livevaccine vector strains. Constructs comprising a 12.2 kb region encodinga consensus promoter and ten contiguous ORF's, and additional flankingDNA were generated which contained all of the information required toproduce the Shigella form I O-Ps antigen. Significantly, attenuatedSalmonella enterica serovar Typhi live vector vaccine candidate strains,containing minimal-sized form I O-Ps operon constructs, elicited immuneprotection in mice against virulent S. sonnei challenge.

SUMMARY

In one aspect of the invention, an immunoprotective compositioncontaining an attenuated bacteria capable of expressing an antigenuseful for inducing an immunoprotective response against Shigella sonnei(S. sonnei) is prepared. The antigen comprises the S. sonnei form IO-polysaccharide and the antigen is produced by enzymes encoded by anexpression cassette containing a nucleotide fragment comprising thegenes wbgT, wbgU, wzx, wzy, wbgV, wbgW, wbgX, wbgY, and wbgZ isolatedfrom the S. sonnei rjb/rfc gene cluster or Plesiomonas shigelloides (P.shigelloides) O17 gene cluster which are operably linked totranscriptional promoter and termination signals. The gene containingfragment is between 10,000 and 13,700 nucleotides in length. Theexpression cassette containing the fragment does not include sequencesthat naturally flank the rfb/rfc gene cluster.

In another aspect of the invention, the attenuated bacteria in theimmunoprotective composition are selected from the group consisting ofCampylobacter jejuni, Campylobacter coli, Listeria monocytogenes,Yersinia enterocolitica, Yersinia pestis, Yersinia pseudotuberculosis,Escherichia coli, Shigella flexneri, Shigella sonnei, Shigelladysemeriae, Shigella boydii, Helicobacter pylori, Helicobacter felis,Gastrospirillum hominus, Vibrio cholerae, Vibrio parahaemdlyticus,Vibrio vulnificus, Bacteroides fragilis, Clostridium difficile,Salmonella typhinntrium, Salmonella typhi, Salmonella gallinatun,Salmonella pullorum, Salmonella choleraesuis, Salmonella enteritidis,Streptococcus gordonii, Lactobacillis sp., Klebsiella pneumoniae,Enterobacter cloacae, and Enterococcus faecalis.

In one embodiment of the invention, the attenuated bacteria are E. colibacteria selected from the group consisting of the strains DH5α andHB101. In another embodiment of the invention, the attenuated bacteriaare S. typhi bacteria selected from the group consisting of the strainsTy21a, CVD 908, CVD 908-htrA, X4073 and Ty800. In a particularlypreferred embodiment, the attenuated S. typhi bacteria are theattenuated strain of Ty2. In another embodiment, the attenuated bacteriaare S. sonnei bacteria selected from the group consisting of strains53GI and 53GII.

In one aspect of the invention, the gene containing fragment in theimmunoprotective composition comprises SEQ ID NO:2, SEQ ID NO:3, or SEQID NO:4 operably linked to a promoter.

In another aspect of the invention, the gene containing fragment of theimmunoprotective composition lacks SEQ ID NO:15.

In still another aspect of the invention the enzymes that produce theantigen are expressed from a recombinant plasmid. In one embodiment ofthe invention, the recombinant plasmid contains a selectable marker. Ina preferred embodiment, the selectable marker is the aspartatef3-semialdehyde dehydrogenase (asd) gene operably linked to a promoter.In still another embodiment of the invention, the enzymes that producethe antigen are encoded by a recombinant plasmid containing SEQ ID NO:2, SEQ ID NO: 3, or SEQ ID NO: 4 operably linked to a promoter. In yetanother embodiment of the invention, the recombinant plasmid lacks SEQTD NO:15. In one embodiment, the recombinant plasmid comprises SEQ IDNO: 2 operably linked to a promoter. In another embodiment, therecombinant plasmid comprises SEQ ID NO: 3 operably linked to apromoter. In still another embodiment, the recombinant plasmid comprisesSEQ ID NO: 4 operably linked to a promoter.

In another aspect of the invention, the immunoprotective compositionalso contains a pharmaceutical diluent.

The invention provides for a method of protecting a susceptible hostagainst an infection of Shigella sonnei (S. sonnei) by administering tothe host an immunoprotective composition containing an attenuatedbacteria capable of expressing an antigen useful for inducing animmunoprotective response against Shigella sonnei (S. sonnei), where theantigen comprises the S. sonnei form I O-polysaccharide, the antigen isproduced by enzymes encoded by an expression cassette containing anucleotide fragment comprising the genes wbgT, wbgU, wzx, wzy, wbgV,wbgW, wbgX, wbgY, and wbgZ isolated from the S. sonnei rfb/rfc genecluster or Plesiomonas shigelloides (P. shigelloides) O17 gene clusterwhich are operably linked to transcriptional promoter and terminationsignals, the gene containing fragment is between 10,000 and 13,700nucleotides in length, the expression cassette containing the fragmentdoes not include sequences that naturally flank the rfb/rfc genecluster, and the composition is given in an amount sufficient to invokean immunoprotective response in the host.

In another aspect the invention provides a method of protecting asusceptible host against an infection of Shigella sonnei (S. swine)comprising administering to said host an immunoprotective compositioncontaining an attenuated bacteria capable of expressing an antigenuseful for inducing an immunoprotective response against Shigella sonnei(S. sonnei), where the antigen comprises the S. sonnei form IO-polysaccharide, the antigen is produced by enzymes encoded by anexpression cassette containing a nucleotide fragment comprising thegenes wbgT, wbgU, wzx, wzy, wbgV, wbgW, wbgX, wbgY, and wbgZ isolatedfrom the S. sonnei rfb/rfc gene cluster or Plesiomonas shigelloides (P.shigelloides) O17 gene cluster which are operably linked totranscriptional promoter and termination signals, the gene containingfragment is between 10,000 and 13,700 nucleotides in length, theexpression cassette containing the fragment does not include sequencesthat naturally flank the rfb/rfc gene cluster, the expression cassetteis on a recombinant plasmid, and the immunogenic composition is given inan amount sufficient to invoke an immunoprotective response in the host.In one embodiment of the method, the enzymes that produce the antigenare encoded by a recombinant plasmid containing SEQ ID NO: 2, SEQ ID NO:3, or SEQ ID NO: 4 operably linked to a promoter. In another embodimentof the method, the immunogenic composition is in a pharmaceuticallyacceptable carrier. In still another embodiment of the method, theimmunogenic composition is in a sterile medium. In another embodiment ofthe method, the immunogenic composition also contains an adjuvant.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1. Cloning and downsizing of the S. sonnei form I biosynthetic genecluster for sequencing and O-antigen expression studies. (A) Restrictionmap of the 30 kb BamH1 insert from cosmid pXG914. (B) The inserts ofplasmid subclones prepared to define a minimal essential region for formI O-antigen expression, defined by anti-form I specific bacterialagglutination of recipient S. sonnei 53GII, E. coli HB101, or S. TyphiTy21a carrying each of these plasmids. (C) Map of the form I gene regionshowing restriction sites relative to inserts shown in panel B and thelocation of 18 ORFs identified by sequence analysis. Filled ORFsrepresent the genes required for form I O-Ps biosynthesis in plasmidbearing subclones. Restriction endonuclease sites are shown for BamHI(B), HindIII (H), PmeI (P), SmaI (S), and XbaI (X). (D) Percent GCcontent of the 17,986 by form I biosynthetic region and flankingsequences.

FIG. 2. Detection of SDS-PAGE separated O-Ps by silver staining andanti-form I Western immunoblotting with form I specific antiserum. O-Psfrom: (A) S. sonnei 53GI, strain 53GII alone (control) or carryingplasmids with different form I-encoding inserts; (B) E. coli HB101 alone(control) or carrying different form I-encoding plasmids; (C) S. typhiTy21a alone (control) or carrying different form I-encoding plasmids.

FIG. 3. ORF diagrams of the regions flanking the S. sonnei form Ibiosynthetic gene cluster. (A) Regions of pWR101 and pWR102 upstream ofwbgT, (B) Region of pWR 101 downstream of wbgZ. The sequences of theleft and right inverted repeats (IRL and IRR) of IS91 are shown in boldtype. The -gttc- target sequence of IS91 is italicized. The original-gttc- sites within IS630 and IS911 for insertion of IS91 are boxed. Asequence homologous to a Pseudomonas IS element occurs within thehatched region.

FIG. 4. Comparison of gene clusters for biosynthesis of the S. sonneiform I O-Ps and the substantially identical P. shigelloides O17 Ps: (A)Composite S. sonnei 53G form I gene cluster and flanking regions derivedfrom Genbank accession numbers AF285971 (SEQ ID NO:16), AF294823 (SEQ IDNO:7) and AF455358 (SEQ ID NO:8). ORFs are identified numerically asdefined in Table 2 and also by gene designations (38). (B) S. sonnei 53Gform I gene cluster reported by Houng and Venkatesan (24). (C) partialS. sonnei HW383 form I gene cluster determined by Chida et al. (6). (D)Composite P. shigelloides O17 Ps gene cluster derived from Genbankaccession numbers AF285970 (SEQ ID NO:17) and AB025970 (SEQ ID NO:18).ORFs are identified numerically and by gene names (38). The ORFs forform I O-Ps biosynthesis by plasmid-bearing subclones are shaded.

FIG. 5. Proposed pathway for biosynthesis of undecaprenyl phosphate(und-P)-linked, S. sonnei form I O-Ps. The pathway is based on thepredicted enzymatic activities of S. sonnei 53G proteins as summarizedin Table 2 and the structural steps required for conversion ofUDP-GlcNAc to the putative form I O-Ps precursors, UDP-L-AltNAcA andUDP-4-n-D-FucNAc.

DEFINITIONS

The term “operon” refers to a cluster of functionally related geneswhose expression or operation is regulated by the same preceedingpromoter gene. The term “rfb/rfc” is the gene symbol for the genecluster which encodes all of the proteins required to synthesize,polymerize, and transport to the bacterial surface the form IO-Polysaccharide of Shigella sonnei. The rfb/rfc gene cluster comprisesthe genes wbgT, wbgU, wzx, wzy, wbgV, wbgW, wbgX, wbgY, and wbgZ (seeTable 2 and SEQ ID NO:7). Included in the cluster but not required forproduction of the form I O-Ps is the transposable element IS630 (SEQ IDNO:15). Also included in the gene cluster are the promoter and operatorsequences (SEQ ID NO:12) for the gene cluster located in thecarboxyterminus of the wzz gene immediately upstream (5′) of the wbgTgene, and the transcriptional terminator sequences are locatedimmediately downstream (3′) of the wbgZ gene (SEQ ID NO:13). Sequenceswhich naturally flank the rfb/rfc gene cluster include those sequencesfound on the S. sonnei virulence plasmid containing the rfb/rfc genecluster not contained in SEQ ID NO:2.

The term “form I O-Polysaccharide” refers to the Shigella sonnei Oantigen composed of disaccharide repealing wills containing two unusualamino sugars, 2-amino-2-deoxy-L-althuronic (L-AItNAcA) and2-acetamido-4-amino-2,4,6-trideoxy-D-galactose (4-nD-FucNAc).

The term “form I O-Ps” is a short hand designation for and usedinterchangeably herein for the Shigella sonnei form I O-Polysaccharidesurface antigen.

The term “O17 gene cluster” is the name of the gene cluster isolatedfrom Plesiomonas shigelloides (P. shigelloides) encoding the genes wbgT,wbgU, wzx, wzy, wbgV, wbgW, wbgX, wbgY, and wbgZ (SEQ ID NO:17). Thegenes are located in an operon on the bacterial chromosome. The O17 genecluster is substantially identical to the rfb/rfc gene cluster. Thenucleotide sequence identity between the clusters ranges from 95% to100% depending on the gene. The amino acid sequence identity ranges from98% to 100%, depending on the gene and the amino acid sequencesimilarity ranges from 99% to 100% depending on the gene. The O17 genecluster lacks the IS630 transposable element found in the rfb/rfc genecluster. The genes encoded by the O17 gene cluster produce the sameenzymes and are capable of producing the same form I O-Ps surfaceantigen as the rfb/rfc gene cluster. Sequences which naturally flank theO17 gene cluster include those sequences found on the P. shigelloidesbacterial chromosome which are not substantially identical the sequencescontained in SEQ ID NO:4.

The term “attenuated,” when used with respect to a bacteria, means thatthe bacteria has lost some or all of its ability to proliferate and/orcause disease or other adverse effect when the bacteria infects anoruanism. For example, an “attenuated” bacteria can be unable toreplicate at all, or be limited to one or a few rounds of replication,when present in an organism in which a wild-type or other pathogenicversion of the attenuated bacteria can replicate. Alternatively oradditionally, an “attenuated” bacteria might have one or more mutationsin a gene or genes that are involved in pathogenicity of the bacteria.Many genes, loci, or operons are known, mutations in which will resultin an attenuated bacteria. Examples of attenuated bacteria used as livevaccines include S. typhi carrying a mutation in its galE or htrA gene,and V. cholerae carrying mutations in its ctxA gene.

A “host organism” is an animal that is a target of vaccination with theattenuated vaccines of the invention. Such host organisms have an immunesystem that is responsive to inoculation with an immunogen. Suitablehost organisms include, for example, humans, rodents, livestock, birds,and other animals in which it is desirable to vaccinate for eithertherapeutic or prophylactic purposes.

The term “vaccine,” is used interchangeably herein with“immunoprotective composition” and as used herein, refers to animmunogen that, upon inoculation into a host organism, can inducecomplete or partial immunity to pathogenic agents, or can reduce theeffects of diseases associated with pathogenic agents.

The terms “nucleic acid” or “oligonucleotide” or grammatical equivalentsherein refer to at least two nucleotides covalently linked together. Theterm nucleic acid is used interchangeably with gene, cDNA, and mRNAencoded by a gene. A nucleic acid of the present invention is preferablysingle-stranded or double stranded and will generally containphosphodiester bonds, although in some cases, as outlined below, nucleicacid analogs are included that may have alternate backbones, comprising,for example, phosphoramide (Beaucage et al. (1993) Tetrahedron49(10):1925) and references therein; Letsinger (1970) J. Org. Chem.35:3800; Sprinzl et al. (1977) Eur. J. Biochem. 81:579; Letsinger et al.(1986) Nucl. Acids Res. 14:3487; Sawai et al. (1984) Chem. Lett. 805,Letsinger et al. (1988) J. Am. Chem. Soc. 110:4470; and Pauwels et al.(1986) Chemica Scripta 26:141 9), phosphorothioate (Mag et al. (1991)Nucleic Acids Res. 19:1437; and U.S. Pat. No. 5,644,048),phosphorodithioate (Briu et al. (1989) J. Am. Chem. Soc. 111:2321,O-methylphosphoroamidite linkages (see Eckstein, Oligonucleotides andAnalogues: A Practical Approach, Oxford University Press), and peptidenucleic acid backbones and linkages (see Egholm (1992) J. Am. Chem. Soc.114:1895; Meier et al. (1992) Chem. Int. Ed. Engl. 31:1008; Nielsen(1993) Nature, 365:566; Carlsson et al. (1996) Nature 380:207). Otheranalog nucleic acids include those with positive backbones (Denpcy etal. (1995) Proc. Natl. Acad. Sci. USA 92:6097; non-ionic backbones (U.S.Pat. Nos. 5,386,023, 5,637,684, 5,602,240, 5,216,141 and 4,469,863;Angew. (1991) Chem. Intl. Ed. English 30:423; Letsinger et al. (1988) J.Am. Chem. Soc. 110:4470; Letsinger et al. (1994) Nucleoside & Nucleotide13:1597; Chapters 2 and 3, ASC Symposium Series 580, “CarbohydrateModifications in Antisense Research”, Ed. Y. S. Sanghui and P. Dan Cook;Mesmaeker et al. (1994), Bioorganic & Medicinal Chem. Lett. 4:395; Jeffset al. (1994) J. Biomolecular NMR 34:17; Tetrahedron Lett. 37:743(1996)) and non-ribose backbones, including those described in U.S. Pat.Nos. 5,235,033 and 5,034,506, and Chapters 6 and 7, ASC Symposium Series580, Carbohydrate Modifications in Antisense Research, Ed. Y. S. Sanghuiand P. Dan Cook. Nucleic acids containing one or more carbocyclic sugarsare also included within the definition of nucleic acids (see Jenkins etal. (1995), Chem. Soc. Rev. pp 169-176). Several nucleic acid analogsare described in Rawls, C & E News Jun. 2, 1997 page 35. Thesemodifications of the ribose-phosphate backbone may be done to facilitatethe addition of additional moieties such as labels, or to increase thestability and half-life of such molecules in physiological environments.

A “exogenous DNA segment”, “heterologous sequence” or a “heterologousnucleic acid”, as used herein, is one that originates from a sourceforeign to the particular host cell, or, if from the same source, ismodified from its original form. Thus, a heterologous gene in a hostcell includes a gene that is endogenous to the particular host cell, buthas been modified. Thus, the terms refer to a DNA segment which isforeign or heterologous to the cell, or homologous to the cell but in aposition within the host cell nucleic acid in which the element is notordinarily found. Exogenous DNA segments are expressed to yieldexogenous polypeptides.

The term “gene” is used broadly to refer to any segment of DNAassociated with a biological function. Thus, genes include codingsequences and/or the regulatory sequences required for their expression.Genes also include nonexpressed DNA segments that, for example, formrecognition sequences for other proteins. Genes can be obtained from avariety of sources, including cloning from a source of interest orsynthesizing from known or predicted sequence information, and mayinclude sequences designed to have desired parameters.

The terms “polypeptide,” “peptide” and “protein” are usedinterchangeably herein to refer to a polymer of amino acid residues. Theterms apply to amino acid polymers in which one or more amino acidresidue is an artificial chemical mimetic of a corresponding naturallyoccurring amino acid, as well as to naturally occurring amino acidpolymers and non-naturally occurring amino acid polymer.

The term “amino acid” refers to naturally occurring and synthetic aminoacids, as well as amino acid analogs and amino acid mimetics thatfunction in a manner similar to the naturally occurring amino acids.Naturally occurring amino acids are those encoded by the genetic code,as well as those amino acids that are later modified, e.g.,hydroxyproline, γ-carboxyglutamate, and O-phosphoserine. Amino acidanalogs refers to compounds that have the same basic chemical structureas a naturally occurring amino acid, i.e., a carbon that is bound to ahydrogen, a carboxyl group, an amino group, and an R group, e.g.,homoserine, norleucine, methionine sulfoxide, methionine methylsulfonium. Such analogs have modified R groups (e.g., norleucine) ormodified peptide backbones, but retain the same basic chemical structureas a naturally occurring amino acid. Amino acid mimetics refers tochemical compounds that have a structure that is different from thegeneral chemical structure of an amino acid, but that functions in amanner similar to a naturally occurring amino acid.

Amino acids may be referred to herein by either their commonly knownthree letter symbols or by the one-letter symbols recommended by theIUPAC-RJB Biochemical Nomenclature Commission. Nucleotides, likewise,may be referred to by their commonly accepted single-letter codes.

“Conservatively modified variants” applies to both amino acid andnucleic acid sequences. With respect to particular nucleic acidsequences, conservatively modified variants refers to those nucleicacids which encode identical or essentially identical amino acidsequences, or where the nucleic acid does not encode an amino acidsequence, to essentially identical sequences. Because of the degeneracyof the genetic code, a large number of functionally identical nucleicacids encode any given protein. For instance, the codons GCA, GCC, GCGand GCU all encode the amino acid alanine. Thus, at every position wherean alanine is specified by a codon, the codon can be altered to any ofthe corresponding codons described without altering the encodedpolypeptide. Such nucleic acid variations are “silent variations,” whichare one species of conservatively modified variations. Every nucleicacid sequence herein which encodes a polypeptide also describes everypossible silent variation of the nucleic acid. One of skill willrecognize that each codon in a nucleic acid (except AUG, which isordinarily the only codon for methionine, and TGG, which is ordinarilythe only codon for tryptophan) can be modified to yield a functionallyidentical molecule. Accordingly, each silent variation of a nucleic acidwhich encodes a polypeptide is implicit in each described sequence.

As to amino acid sequences, one of skill will recognize that individualsubstitutions, deletions or additions to a nucleic acid, peptide,polypeptide, or protein sequence which alters, adds or deletes a singleamino acid or a small percentage of amino acids (typically less than 5%,more typically less than 1%) in the encoded sequence is a“conservatively modified variant” where the alteration results in thesubstitution of an amino acid with a chemically similar amino acid.Conservative substitution tables providing functionally similar aminoacids are well known in the art. Such conservatively modified variantsare in addition to and do not exclude polymorphic variants, interspecieshomologs, and alleles of the invention.

The following eight groups each contain amino acids that areconservative substitutions for one another:

1) Alanine (A), Glycine (G);

2) Aspartic acid (D), Glutamic acid (E);

3) Asparagine (N), Glutamine (Q);

4) Arginine (R), Lysine (K);

5) Isoleucine (I), Leucine (L), Methionine (M), Valine (V);

6) Phenylalanine (F), Tyrosine (Y), Tryptophan (W);

7) Serine (S), Threonine (T); and

8) Cysteine (C), Methionine (M)

(see, e.g., Creighton, Proteins (1984)).

Macromolecular structures such as polypeptide structures can bedescribed in terms of various levels of organization. For a generaldiscussion of this organization, see, e.g., Alberts et al., MolecularBiology of the Cell (3.sup.rd ed., 1994) and Cantor and Schimmel,Biophysical Chemistry Part I: The Conformation of BiologicalMacromolecules (1980). “Primary structure” refers to the amino acidsequence of a particular peptide. “Secondary structure” refers tolocally ordered, three dimensional structures within a polypeptide.These structures are commonly known as domains. Domains are portions ofa polypeptide that form a compact unit of the polypeptide and aretypically 15 to 350 amino acids long. Typical domains are made up ofsections of lesser organization such as stretches of β-sheet andα-helices. “Tertiary structure” refers to the complete three dimensionalstructure of a polypeptide monomer. “Quaternary structure” refers to thethree dimensional structure formed by the noncovalent association ofindependent tertiary units.

The term “isolated”, when applied to a nucleic acid or protein, denotesthat the nucleic acid or protein is essentially free of other cellularcomponents with which it is associated in the natural state. It ispreferably in a homogeneous state although it can be in either a dry oraqueous solution. Purity and homogeneity are typically determined usinganalytical chemistry techniques such as polyacrylamide gelelectrophoresis or high performance liquid chromatography. A proteinwhich is the predominant species present in a preparation issubstantially purified. In particular, an isolated gene is separatedfrom open reading frames which flank the gene and encode a protein otherthan the gene of interest. The term “purified” denotes that a nucleicacid or protein gives rise to essentially one band in an electrophoreticgel. Particularly, it means that the nucleic acid or protein is at leastabout 50% pure, more preferably at least about 85% pure, and mostpreferably at least about 99% pure.

The term “naturally-occurring” is used to describe an object that can befound in nature as distinct from being artificially produced by man. Forexample, an organism, or a polypeptide or polynucleotide sequence thatis present in an organism (including viruses, bacteria, protozoa,insects, plants or mammalian tissue) that can be isolated from a sourcein nature and which has not been intentionally modified by man in thelaboratory is naturally-occurring.

“Nucleic acid derived from a gene” refers to a nucleic acid for whosesynthesis the gene, or a subsequence thereof, has ultimately served as atemplate. Thus, an mRNA, a cDNA reverse transcribed from an mRNA, an RNAtranscribed from that cDNA, a DNA amplified from the cDNA, an RNAtranscribed from the amplified DNA, etc., are all derived from the geneand detection of such derived products is indicative of the presenceand/or abundance of the original gene and/or gene transcript in asample.

A nucleic acid is “operably linked” when it is placed into a functionalrelationship with another nucleic acid sequence. For instance, apromoter or enhancer is operably linked to a coding sequence if itincreases the transcription of the coding sequence. Operably linkedmeans that the DNA sequences being linked are typically contiguous and,where necessary to join two protein coding regions, contiguous and inreading frame. However, since enhancers generally function whenseparated from the promoter by several kilobases and intronic sequencesmay be of variable lengths, some polynucleotide elements may be operablylinked but not contiguous.

The term “recombinant” when used with reference to a bacteria indicatesthat the host bacteria contains a heterologous nucleic acid, orexpresses a peptide or protein encoded by a heterologous nucleic acid.Heterologous nucleic acids can integrate into the host bacteriachromosome and be expressed from host or heterologous promoters.Alternatively, heterologous nucleic acids can be expressed from anautonomously replicating plasmid. Recombinant bacteria can contain genesthat are not found within the native (non-recombinant) form of thebacteria. Recombinant bacteria can also contain genes found in thenative form of the bacteria wherein the genes are modified andre-introduced into the cell by artificial means. The term alsoencompasses bacteria that contain a nucleic acid endogenous to thebacteria that has been modified without removing the nucleic acid fromthe bacteria; such modifications include those obtained by genereplacement, site-specific mutation, and related techniques.

A “recombinant expression cassette” or simply an “expression cassette”is a nucleic acid construct, generated recombinantly or synthetically,with nucleic acid elements that are capable of effecting expression of.a structural gene in hosts compatible with such sequences. Expressioncassettes include at least promoters and optionally, transcriptiontermination signals. Typically, the recombinant expression cassetteincludes a nucleic acid to be transcribed (e.g., a nucleic acid encodinga desired polypeptide or series of peptides), and a promoter. Additionalfactors necessary or helpful in effecting expression may also be used asdescribed herein. For example, an expression cassette can also includenucleotide sequences that encode a signal sequence that directssecretion of an expressed protein from the host cell. Transcriptiontermination signals, enhancers, and other nucleic acid sequences thatinfluence gene expression, can also be included in an expressioncassette. The recombinant expression cassette may be located on anautonomously replicating plasmid or may be integrated into the hostgenome.

The term “selectable marker” refers to a nucleotide sequence thatencodes a protein and that confers either a positive or negativeselective advantage to a bacteria expressing that marker. For example,an expression cassette comprising a selectable marker could comprise theaspartate β-semialdehyde dehydrogenase (asd) gene operably linked to apromoter. A recombinant plasmid capable of expressing asd couldcomplement the asd phenotype of asd deletion mutants. Bacteria lackingasd would not be able to synthesize diaminopimelic acid, an essentialelement of the peptidoglycan of the bacterial cell wall, and would die.Examples of other selectable markers useful in bacteria include SacB,aroA, and heavy metal ion resistance genes.

The terms “identical” or percent “identity,” in the context of two ormore nucleic acid or polypeptide sequences, refer to two or moresequences or subsequences that are the same or have a specifiedpercentage of amino acid residues or nucleotides that are the same, whencompared and aligned for maximum correspondence, as measured using oneof the following sequence comparison algorithms or by visual inspection.

The phrase “substantially identical,” in the context of two nucleicacids or polypeptides, refers to two or more sequences or subsequencesthat have at least 60%, preferably 80%, most preferably 90-95%nucleotide or amino acid residue identity, when compared and aligned formaximum correspondence, as measured using one of the following sequencecomparison algorithms or by visual inspection. Preferably, thesubstantial identity exists over a region of the sequences that is atleast about 50 residues in length, more preferably over a region of atleast about 100 residues, and most preferably the sequences aresubstantially identical over at least about 150 residues. In a mostpreferred embodiment, the sequences are substantially identical over theentire length of the coding regions.

For sequence comparison, typically one sequence acts as a referencesequence to which test sequences are compared. When using a sequencecomparison algorithm, test and reference sequences are input into acomputer, subsequence coordinates are designated, if necessary, andsequence algorithm program parameters are designated. The sequencecomparison algorithm then calculates the percent sequence identity forthe test sequence(s) relative to the reference sequence, based on thedesignated program parameters.

Optimal alignment of sequences for comparison can be conducted, e.g., bythe local homology algorithm of Smith & Waterman, Adv. Appl. Math. 2:482(1981), by the homology alignment algorithm of Needleman & Wunsch, J.Mot. Biol. 48:443 (1970), by the search for similarity method of Pearson& Lipman, Proc. Nat'l. Acad. Sci. USA 85:2444 (1988), by computerizedimplementations of these algorithms (GAP, BESTFIT, FASTA, and TFASTA inthe Wisconsin Genetics Software Package, Genetics Computer Group, 575Science Dr., Madison, Wis.), or by visual inspection (see generallyAusubel et al., Current Protocols in Molecular Biology, John Wiley andSons Inc. New York, N.Y. (2001)).

One example of an algorithm that is suitable for determining percentsequence identity and sequence similarity is the BLAST algorithm, whichis described in Altschul et al., J. Mol. Biol: 215:403-410 (1990).Software for performing BLAST analyses is publicly available through theNational Center for Biotechnology Information(http://www.ncbi.nlm.nih.gov/).

A “subsequence” refers to a sequence of nucleic acids or amino acidsthat comprise a part of a longer sequence of nucleic acids or aminoacids (e.g., polypeptide) respectively.

The term “protective immunity” means that a vaccine or immunizationschedule that is administered to a mammal induces an immune responsethat prevents, retards the development of, or reduces the severity of adisease or infection that is caused by Shigella sonnei, or diminishes oraltogether eliminates the symptoms of the disease or infection.

The phrase “sufficient to invoke an immunoprotective response” meansthat there is a detectable difference between an immune responseindicator measured before and after administration of a particularantigen preparation. Immune response indicators include but are notlimited to: antibody titer or specificity, as detected by an assay suchas enzyme-linked immunoassay (ELISA), bactericidal assay, flowcytometry, immunoprecipitation, Ouchter-Lowny immunodiffusion; bindingdetection assays of, for example, spot, Western blot or antigen arrays;cytotoxicity assays, etc.

A “surface antigen” is an antigen that is present in a surface structureof a bacteria such as the Shigella sonnei form I O-Ps antigen, which iscapable of generating an immunoprotective response when expressed by arecombinant bacteria and presented to a host organism in animmunoprotective composition.

DETAILED DESCRIPTION

This invention is directed to a living, attenuated, oral vaccine capableof inducing an immunoprotective response against Shigella sonnei. Theinvention is based on an attenuated strain of bacteria which has beengenetically engineered to carry the genes encoding the enzymes capableof synthesizing the S. sonnei form I O-Ps antigen. These recombinantbacteria are useful in an immunoprotective composition to induce animmunoprotective response in a susceptible host organism. In addition toinfections caused by S. sonnei, enteric infections caused by otherorganisms are considered amenable to treatment with a combinationvaccine according to this invention. For example, genes encoding thesurface antigens derived from other Shigella strains such as S.flexneri, S. dysenteriae, and S. boydii (see e.g., Baron et al., Infect.and Immun. 55:2797 (1987)) can be transferred into recipient bacteriaindependently of or concurrently with the S. sonnei rfb/rfc genecluster. The resulting recombinant bacteria can then express two or moreheterologous surface antigens suitable for generating animmunoprotective response in a host organism. Alternatively, the oralvaccine may contain multiple strains of attenuated bacteria, each strainexpressing a different heterologous antigen. This resulting vaccinewould also be suitable for generating an immunoprotective responseagainst multiple antigens in a host organism.

Genes encoding other antigens, such as Salmonella typhi Vi antigen andgenes encoding non-toxic variants of toxins derived from enterotoxogenicstrains such as Escherichia coli, Vibrio cholera, and Yersinia can alsobe transferred independently of or concurrently with the S. sonneirfb/rfc gene cluster into bacterial hosts (see e.g. U.S. Pat. No.4,632,830). In a preferred embodiment, the Vi antigen or non-toxicvariants of the enterotoxins should be expressed in such a way that theproteins are present on the surface of the recombinant bacteria orsecreted by the recombinant bacteria. The resulting recombinant bacteriawould be useful in immunogenic compositions for generating animmunoprotective response to these additional antigens. Enteric diseasecaused by bacterial secretion of an exotoxin exemplified bystaphylococcal, clostridial or similar food poisoning are alsoconsidered amenable to treatment with an immunoprotective compositionaccording to this invention using an approach similar to the approachused for enterotoxins.

Nucleic acids encoding the S. sonnei rfb/rfc gene cluster as exemplifiedin SEQ ID NO:2-4, the O17 gene cluster, or other antigens are typicallycloned into vectors for transformation into bacterial cells forreplication, expression, and cell transformation. Such vectors aretypically prokaryotic vectors, e.g., plasmids that act as shuttlevectors, or for production of protein. The elements that are typicallyincluded in vectors include a replicon that functions in the recombinantbacteria, a gene encoding a selectable marker to permit selection ofbacteria that harbor recombinant plasmids, and unique restriction sitesin nonessential regions of the plasmid to allow insertion of recombinantsequences. Selectable markers may include a gene encoding antibioticresistance, or may include a gene encoding a protein whose naturallyoccurring gene has been mutated resulting in an attenuated strain ofbacteria. Examples of suitable targets for mutation include genes thatwould result in essential auxotrophic pathways, loci encoding regulonsthat exert pleiotropic effects such as the cya/crp system, theompft/envZ system or the phoP system (see e.g. U.S. Pat. Nos. 5,672,345,5,980,907, 6,190,669). A preferred selectable marker is the aspartateβ-semialdehyde dehydrogenase (asd) gene operably linked to a promoter. Arecombinant plasmid capable of expressing asd could complement the asdphenotype of attenuated bacterial strains suitable for use in vaccinesand containing asd deletion mutantations. Bacteria lacking asd would notbe able to synthesize diaminopimelic acid, an essential element of thepeptidoglycan of the bacterial cell wall, and would die. Examples ofother selectable markers useful in bacteria include SacB, aroA, andheavy metal ion resistance genes.

Alternatively, vectors containing nucleic acids encoding the enzymesthat produce the form I O-Ps antigen may be transformed into bacterialcells carrying a mutation in the msbB gene. Mutations in this gene failto myristylate lipid A. Bacteria containing this mutation may containadditional mutations resulting in attenuated bacteria and vectorscontaining the enzymes that produce the form I O-Ps may containselectable markers. Form I O-Ps produced in bacteria containing amutation in the msbB gene may be purified using techniques well known tothose of skill in the art and used in an immunoprotective compositiondirectly.

To obtain expression of the S. sonnei rfb/rfc gene cluster, the O17 genecluster, or other antigens, the nucleic acids encoding the appropriategene(s) are typically subcloned using techniques well known to those ofskill in the art, into an expression vector that contains a promoter todirect transcription. Suitable bacterial promoters are well known in theart and described, e.g., in Sambrook et al., Molecular Cloning, ALaboratory Manual (2nd ed. 1989); Kriegler, Gene Transfer andExpression: A Laboratory Manual (1990); and Current Protocols inMolecular Biology (Ausubel et al., eds., 2001).

The promoter used to direct expression of the S. sonnei rfb/rfc genecluster, the O17 gene cluster, or other antigen depends on theparticular application. Either a constitutive or an inducible promotermay be used. Preferably, a constitutive promoter is used. Alternatively,the promoter which drives the normal expression of the S. sonnei rfb/rfcgene cluster can be used.

The promoter typically can also include elements that are responsive totransactivation, e.g., hypoxia response elements, Ga14 responseelements, lac repressor response element, and small molecule controlsystems such as tet-regulated systems and the like (see, e.g., Gossen &Bujard, Proc. Nat'l Acad. Sci. USA 89:5547 (1992); Oligino et al., GeneTher. 5:491-496 (1998); Wang et al., Gene Ther. 4:432-441 (1997);Neering et al., Blood 88:1147-1155 (1996); and Rendahl et al., Nat.Biotechnol. 16:757-761 (1998)).

In addition to the promoter, the expression vector typically contains atranscription unit or expression cassette that contains all theadditional elements required for the expression of the nucleic acid inrecombinant bacteria. A typical expression cassette thus contains apromoter operably linked, e.g., to the nucleic acid sequence encodingthe rfb/rfc gene cluster, and signals required, e.g., fortranscriptional termination, ribosome binding sites, or translationtermination. Additional elements of the cassette may include, e.g.,regulatory proteins.

Standard bacterial vectors include plasmids such as pBR322 basedplasmids, pBR325, pUC18, pSKF, pET23D, and pBR322 based cosmid vectorssuch as pHC79 and pCVD551. Vectors based on the bacterial plasmid pSC101such as pGB-2 may also be used.

Standard transformation methods are used to produce bacterial cell linesthat express the surface antigen proteins of the invention.Transformation of prokaryotic cells are performed according to standardtechniques (see, e.g., Morrison, J. Bact. 132:349-351 (1977); Sambrooket al. supra; Ausubel et al. supra). These methods includemicroinjection, ballistics, use of calcium chloride transformation,infection, conjugation, and electroporation of plasmid vectors, bothepisomal and integrative, and any of the other well known methods forintroducing cloned genomic DNA, synthetic DNA or other foreign geneticmaterial into a recombinant bacteria (see, e.g., Sambrook et al., supra,see also U.S. Pat. No. 5,049,386, U.S. Pat. No. 4,946,787; U.S. Pat. No.4,897,355; WO 91/17424, and WO 91/16024). It is only necessary that theparticular genetic engineering procedure used be capable of successfullyintroducing at least one gene into the recombinant bacteria capable ofexpressing the protein of choice.

The microorganisms which are used to express the S. sonnei rfb/rfc genecluster, the O17 gene cluster and other antigens for use inimmunoprotective compositions include without limitation, Campylobactersp., Yersinia sp., Helicobacter sp., Gastrospirillum sp., Bacteroidessp., Klebsiella sp., Laclobacillis sp., Streptococcus gordonii,Enterobacter sp., Salmonella sp., Shigella sp., Aeromonas sp., Vibriosp., Clostridium sp., Emerococcus sp. and Escherichia coli (see e.g.U.S. Pat. Nos. 5,858,352, and 6,051,416, and Levine et al., in “NewGeneration Vaccines Second Edition” ed. Levine et al., Marcel Dekker,Inc. pp 351-361 (1997), Levine et al., in “New Generation VaccinesSecond Edition” ed. Levine et al., Marcel Dekker, Inc. pp 437-446(1997), Butterton et al., in “New Generation Vaccines Second Edition”ed. Levine et al., Marcel Dekker, Inc. pp 379-385 (1997) and Fennelly etal., in “New Generation Vaccines Second Edition” ed. Levine et al.,Marcel Dekker, Inc. pp 363-377 (1997)).

Preferred enteric bacteria that the various aspects of the presentinvention relate to are Campylobacter jejuni, Campylobacter coli,Listeria monocytogenes, Yersinia enterocolitica, Yersinia pestis,Yersinia pseudotuberculosis, Escherichia coli, Shigella flexneri,Shigella sonnei, Shigella dysenteriae, Shigella boydii, Helicobacterpylori, Helicobacter fells, Gastrospirillum hominus, Vibrio cholerae,Vibrio parahaemolyticus, Vibrio vulnificus, Bacteroides fragilis,Clostridium difficile, Salmonella typhimurium, Salmonella typhi,Salmonella gallinarum, Salmonella pulloruin, Salmonella choleraesuk,Salmonella enteritidis, Klebsiella pneumoniae, Enterobacter cloacae, andEnterococcus faecalis. Preferred Escherichia coli include but are notlimited to entero-toxic, entero-hemorrhagic, entero-invasive,entero-pathogenic or other strains.

More preferred strains of Escherichia coli include DH5α and HB101. Morepreferred strains of Salmonella typhi include CVD 908, CVD 908-htrA,X4073 and TY800. More preferred strains of Shigella sonnei include 53G1and 53 GII.

Most preferred strains of bacteria to use as live attenuated vaccinesinclude S. typhi, strain Ty21a, which carries a mutation in its galEgene, and V. cholerae carrying mutations in its ctxA gene which preventthe expression of cholera toxin.

Attenuated vaccines can be administered directly to the mammal. Thevaccines obtained using the methods of the invention can be formulatedas pharmaceutical compositions for administration in any suitablemanner. The preferred route of administration is oral. Other routes ofadministration include rectal, intrathecal, buccal (e.g., sublingual)inhalation, intranasal, and transdermal (see e.g. U.S. Pat. No.6,126,938). Although more than one route can be used to administer aparticular composition, a particular route can often provide a moreimmediate and more effective reaction than another route.

The immunoprotective compositions to be administered are provided in apharmaceutically acceptable solution such as an aqueous solution, oftena saline or buffered solution, or they can be provided in powder form.There is a wide variety of suitable formulations of pharmaceuticalcompositions of the present invention. See, e.g., Lieberman,Pharmaceutical Dosage Forms, Marcel Dekker, Vols. 1-3 (1998);Remington's Pharmaceutical Science, 17th ed., Mack Publishing Company,Easton, Pa. (1985) and similar publications. The compositions may alsoinclude an adjuvant. Examples of known suitable adjuvants include alum,aluminum phosphate, aluminum hydroxide, and MF59 (4.3% w/v squalene,0.5% w/v Tween 80, 0.5% w/v Span 85)—these are the only ones currentlylicensed for use in humans. For experimental animals, one can useFreund's, N-acetyl-muramyl-L-threonyl-D-isoglutamine (thr-MDP),N-acetyl-nor-muramyl-L-alanyl-D-isoglutamine (CGP 11637, referred to asnor-MDP),N-acetylmuramyl-L-alanyl-D-isoglutaminyl-L-alanine-2-(1′-2′-dip-almitoyl-sn-glycero-3-hydroxyphosphoryloxy)-ethylamine (CGP 19835A, referred to asMTP-PE), and RIBI, which contains three components extracted frombacteria, monophosphoryl lipid A, trehalose dimycolate and cell wallskeleton (MPL+TDM+CWS) in a 2% squalene/Tween 80 emulsion, or BacilleCalmette-Guerin (BCG). The effectiveness of an adjuvant may bedetermined by measuring the amount of antibodies directed against theimmunogenic antigen.

The concentration of immunogenic antigens of the invention in thepharmaceutical formulations can vary widely, i.e. from less than about0.1%, usually at or at least about 2% to as much as 20% to 50% or moreby weight, and will be selected primarily by fluid volumes, viscosities,etc., in accordance with the particular mode of administration selected.

Formulations suitable for oral administration can consist of (a) liquidsolutions, such as an effective amount of the recombinant bacteriasuspended in diluents, such as buffered water, saline or PEG 400; (b)capsules, sachets or tablets, each containing a predetermined amount ofthe active ingredient, as lyophilized powder, liquids, solids, granulesor gelatin; (c) suspensions in an appropriate liquid; and (d) suitableemulsions. Tablet forms can include one or more of lactose, sucrose,mannitol, sorbitol, calcium phosphates, corn starch, potato starch,tragacanth, microcrystalline cellulose, acacia, gelatin, colloidalsilicon dioxide, croscarmellose sodium, talc, magnesium stearate,stearic acid, and other excipients, colorants, fillers, binders,diluents, buffering agents, moistening agents, preservatives, flavoringagents, dyes, disintegrating agents, and pharmaceutically compatiblecarriers. Lozenge forms can comprise the active ingredient in a flavor,usually sucrose and acacia or tragacanth, as well as pastillescomprising the active ingredient in an inert base, such as gelatin andglycerin or sucrose and acacia emulsions, gels, and the like containing,in addition to the active ingredient, carriers known in the art. It isrecognized that the attenuated vaccines, when administered orally, mustbe protected from digestion. This is typically accomplished either bycomplexing the vaccines with a composition to render it resistant toacidic and enzymatic hydrolysis or by packaging the vaccines in anappropriately resistant carrier such as a liposome or enteric coatedcapsules. Means of protecting the attenuated bacteria from digestion arewell known in the art. The pharmaceutical compositions can beencapsulated, e.g., in liposomes, or in a formulation that provides forslow release of the active ingredient.

The attenuated vaccines, alone or in combination with other suitablecomponents, can be made into aerosol formulations (e.g., they can be“nebulized”) to be administered via inhalation. Aerosol formulations canbe placed into pressurized acceptable propellants, such asdichlorodifluoromethane, propane, nitrogen, and the like.

The dose administered to a patient, in the context of the presentinvention should be sufficient to effect a beneficial therapeutic and/orprophylactic response in the patient over time. The dose will bedetermined by the efficacy of the particular attenuated vaccine employedand the condition of the patient, as well as the body weight or vascularsurface area of the patient to be treated. The size of the dose alsowill be determined by the existence, nature, and extent of any adverseside-effects that accompany the administration of a particular vaccinein a particular patient.

In determining the effective amount of the vaccine to be administered inthe treatment or prophylaxis of an infection or other condition, thephysician evaluates vaccine toxicities, progression of the disease, andthe production of anti-vaccine vector antibodies, if any.

The compositions are administered to an animal that is at risk fromacquiring an infection caused by S. sonnei or to prevent or at leastpartially arrest the development of the infection and its complications.An amount adequate to accomplish this is defined as a “therapeuticallyeffective dose.” Amounts effective for therapeutic use will depend on,e.g., the antigen composition, the manner of administration, the weightand general state of health of the patient, and the judgment of theprescribing physician. Single or multiple doses of the antigencompositions may be administered depending on the dosage and frequencyrequired and tolerated by the patient, and route of administration.

In particular embodiments, a therapeutically effective dose of theimmunoprotective composition is administered to an individual. Amountsof live attenutated bacteria expressing the S. sonnei rfb/rfc genecluster (SEQ ID NOs: 2-4), the O17 gene cluster, or other antigenspresent in the initial immunization generally range from about 5×10⁶ to5×10¹¹ organisms per patient, and more commonly from about 5×10⁸ to5×10⁹ organisms per patient.

The existence of an immune response to the first dose of theimmunoprotective composition may be determined by known methods (e.g. byobtaining serum from the individual before and after the initialimmunization, and demonstrating a change in the individual's immunestatus, for example an immunoprecipitation assay, or an ELISA, or abactericidal assay, or a Western blot, or flow cytometric assay, or thelike) prior to administering a subsequent dose. The existence of animmune response to the first dose may also be assumed by waiting for aperiod of time after the first immunization that, based on previousexperience, is a sufficient time for an immune response and/or primingto have taken place—e.g. 1, 2, 4, 6, 10 or 14 weeks. Boosting dosages ofthe immunoprotective composition will contain from about 5×10⁶ to 5×10¹¹organisms per patient, depending on the nature of the immunogen.

The immunoprotective compositions are typically administered to anindividual that is immunologically naive with respect to Shigellasonnei. In a particular embodiment, the individual is a human childabout 1-4 years of age or younger, and the antigen compositions areadministered preferrably at 12 months of age with booster doses givenapproximately one week apart. Usually, 2-4 doses may be sufficient,however additional doses may be required to achieve a high level ofimmunity. Additional booster doses may be given every 1-5 years, asnecessary, to maintain a high level of immunity.

In general, administration to any individual should begin prior to thefirst sign of disease, or possibly at the first sign of possible oractual exposure to Shigella.

The toxicity and therapeutic efficacy of the attenuated vaccinesprovided by the invention are determined using standard pharmaceuticalprocedures in cell cultures or experimental animals. One can determinethe ED₅₀ (the dose therapeutically effective in 50% of the population)using procedures presented herein and those otherwise known to those ofskill in the art.

The attenuated vaccines of the invention can be packaged in packs,dispenser devices, and kits for administering genetic vaccines to amammal. For example, packs or dispenser devices that contain one or moreunit dosage forms are provided. Typically, instructions foradministration of the compounds will be provided with the packaging,along with a suitable indication on the label that the compound issuitable for treatment of an indicated condition. For example, the labelmay state that the active compound within the packaging is useful fortreating a particular infectious disease, enteric disorder, or forpreventing or treating other diseases or conditions that are mediatedby, or potentially susceptible to, a mammalian immune response.

EXAMPLES

The following example is offered to illustrate, but not to limit theclaimed invention.

1. Materials and Methods

Bacterial Strains, Plasmids, and Growth Conditions.

The bacterial strains and plasmids utilized are described in Table 1.Wild type S. sonnei strain 53G form I (i.e. 53GI), harboring the 180 kbvirulence plasmid, was used for cloning studies and as a positivecontrol for LPS analysis and immunoblot assays. Studies of plasmid-basedform I O-Ps expression were performed in E. coli strains HB101 or DH5α,Salmonella serovar Typhi strain Ty21a, and virulence plasmid-minus S.sonnei strain 53G form II (i.e. 53GII).

Cosmids pHC79 and pCVD551 (kindly provided by Timothy McDaniel, Centerfor Vaccine Development, University of Maryland, Baltimore, Md.) wereemployed to clone segments of the 180 kb plasmid of S. sonnei 53GI.Plasmid vectors pBR325, pGB2 and pUCl 8 were used for subcloning.

Bacterial strains were grown at 37° C. in Luria-Bertani broth (LB) or onLB agar (Difco). Plasmid-containing strains were selected in mediacontaining ampicillin (Ap, 100 μg/ml), spectinomycin (Sp, 50 μg/ml),chloramphenicol (Cm, 35 μg/ml), or tetracycline (Tc, 20 μg/ml).

Plasmid Manipulations.

Unless otherwise noted all DNA manipulations were performed essentiallyfollowing the procedures outlined in Sambrook et al. (35). Restrictionenzymes were used with the buffers supplied by the manufacturer (Roche).Electroporation of plasmid constructs was performed with a Gene Pulser(Bio-Rad).

Cloning of S. sonnei Form I Genes.

pWR101 and pWR102 are form I antigen-expressing cosmids that containlarge overlapping regions of the S. sonnei 180 kb plasmid from strain53GI (D. J. Kopecko, L. S. Baron, T. L. Hale, S. B. Formal and K. Noon,Abstr. 83th Annual Meeting of the American Society for Microbiology,abstr. D 10, 1983). These recombinant cosmids, initially selected in E.coil recipients on antibiotic-containing media, were identified bycolony immunoblotting and bacterial agglutination assays using purifiedform I O-antigen-specific, rabbit polyclonal antiserum (see below). Theessential form I genes and flanking sequences were subcloned from the 39kb insert of pWR101 (Table 1). First, pWR101 DNA was digested with BamHland a resulting 30 kb fragment was ligated to the isoschizomerBgIII-digested cosmid pCVD551. DNA was packaged in lambda phageparticles in vitro using a commercial kit (Gigapack plus, Stratagene)according to the manufacturer's instructions. Lambda-packaged DNA wasused to infect E. coil HB101 or DH5a, and the recombinants were screenedfor form 1 antigen expression by colony immunoblotting. A HindIIIpartial digest of one form I-expressing clone, designated pXG914, wasligated to the multicopy plasmids pUC18 and pBR325, and the low copyplasmid pGB-2 (7). Inserts representing one or more of three contiguousHindIII fragments of 12.4, 1.2 and 2.1 kb were initially obtained (i.e.pXK67 (comprising SEQ ID NO:1), pXK68 (comprising SEQ ID NO:1), pXK66(comprising SEQ ID NO:2), pXK65 (comprising SEQ ID NO:2) and pXK46(comprising SEQ ID NO:5)). Additional deletion derivatives (i.e. pXK45(comprising SEQ ID NO:3), pXK50 (comprising SEQ ID NO:4) and pXK47(comprising SEQ ID NO:6)) of this -region were obtained to delimit theform I biosynthetic region (Table 1).

DNA Sequencing and Analysis.

DNA sequencing was performed using Ready Reactions DyeDeoxy Terminatorcycle sequencing kits (Applied Biosystems) and an ABI model 373Aautomated sequencer. Subclones used for sequencing studies includedpXK2.1 (comprising SEQ ID NO:9), pXK1.2 (comprising SEQ ID NO:10),pXK1.4 (comprising SEQ ID NO:11), pXK47 and pXG914 (Table 1). Limitedsequencing of pWRIO2 was also performed. Sequences were assembled andanalyzed using the Vector NTI suite 6.0 software (InforMax, Inc.). DNAhomology searches were performed using the Basic Local Alignment SearchTool (BLAST) of the National Center for Biotechnology Information. TheGenBank sequence accession number for the 17,986 by sequence of pWR101identified in this work is AF294823 (comprising SEQ ID NO:7) and theaccession number for the 2,964 by sequence of pWR102 is AF455358(comprising SEQ ID NO:8).

Antisera and Slide Agglutination.

Rabbit polyclonal form I specific antiserum, kindly provided by S.Formal (Walter Reed Army Institute of Research, Washington, D.C.), wasproduced by repeated immunization of New Zealand white rabbits withwhole cells of heat-killed S. sonnei 53G1. Group D-specific Shigellatyping sera (Difco) was also utilized. These rabbit antisera wereabsorbed with heat-treated (70° C., 30 min) S. sonnei form II and E.coil HB101 cells. Packed cells (0.1 ml) were added to 1.0 ml ofundiluted or 10-fold diluted antiserum, mixed and incubated for 2 h at37° C. and overnight at 4° C. Following centrifugation, the absorbedantiserum was stored at 4° C. for use in bacterial agglutination assaysperformed on microscope slides as previously described (1,2). Absorbedform I-specific antiserum did not agglutinate E. coli, S. sonnei 53GIIor Salmonella host strains.

LPS and Immintoblot Analyses.

Salmonella, Shigella, and E. coli strains carrying various plasmidconstructs were grown overnight with aeration at 37° C. in LB mediacontaining appropriate antibiotics. Bacteria were pelleted bycentrifugation and lysed in SDS-PAGE sample buffer containing 4%2-mercaptoethanol. The sample was boiled for 5 min, treated withproteinase K for 1 h and analyzed by SDS-PAGE using a 15% gel and theLaemmli buffer system (28). Gels were silver-stained (22) or subjectedto Western blotting with form I-specific antiserum.

Western blotting was performed using PVDF membranes (Schleicher &Schuell). The membranes were blocked with 5% non-fat dry milk inTris-buffered saline (TBS; 20 mM Tris-HCl, 150 mM NaCl, pH7.5) andreacted with anti-form I serum followed by protein A-alkalinephosphatase conjugate. The developing solution consisted of 200 mg ofFast Red TR salt and 100 mg of Naphthol NS-MX phosphate (Sigma) in 50 mlof 50 mM Tris buffer (pH 8.0).

Recombinant clones expressing the S. sonnei O-Ps were identified bycolony immunoblotting performed with anti-form I serum and proteinA-alkaline phosphatase conjugate as described above. Colonies ofrecipient E. coil, S. sonnei 53G II, or S. Typhi strains alone did notreact with the absorbed form I-specific antisera under these conditions.

Stability of Form I O-Ps Expression in Salmonella.

Several S. Typhi Ty21a strains, each containing a different formI-expressing recombinant plasmid, were tested for stability of form IO-Ps expression. Each form I-expressing strain was diluted toapproximately 100 cfu per ml and grown for 12 h (i.e. approximately 25generations) with aeration at 37° C. in LB media under nonselectiveconditions (i.e. without antibiotics). These cultures were diluted againto 100 cfu per ml in LB and grown for an additional 12 h. Samples takenafter 12 and 24 h of nonselective growth were plated onto LB agarwithout antibiotics and incubated at 37° C. At least 100 colonies ofeach strain were tested at each time point for O-Ps expression by thecolony immunoblot assay.

Animal Immunization Study.

Outbred ICR mice weighing from 13 to 15 g were used to assess immuneprotection as described previously (12). Vaccine candidate strains andcontrol Ty21a alone were grown overnight in BI-11 broth (Difco)supplemented with 0.01% galactose, washed, and suspended in sterilesaline to a concentration of 5x10⁷ cfu per ml. Mice were inoculatedintraperitoneally with a single 0.5 ml dose of either vaccine or controlcell suspensions or sterile saline. Immunized and control mice werechallenged 5 weeks postimmunization with 5×10⁵ cfu (approximately100xLD₅₀) of freshly grown, mid-log phase S. sonnei strain 53GI in 0.5ml of 5% hog gastric mucin (Sigma) in sterile saline. Survival wasmonitored for 96 h.

II. Results

Cloning and Genetic Downsizing for Expression of the Form I O-AntigenLocus.

To delimit the DNA region required for biosynthesis of form I antigen,we initially cloned this region from S. sonnei strain 53G1 in cosmids(see Methods). The 30 kb BamH1 insert of pXG914, which directs theexpression of typical form I LPS in E. coli, was partially digested withHindl1.l and separately ligated to low and high copy plasmid vectorspGB-2, pBR325, and pUC18. The resulting form I-expressing subclones(Table 1), containing inserts comprised of one or more of three adjacentHindIII fragments of 12.4, 1.2, and 2.1 kb (e.g. pXK67, pXK65, and pXK46and several additional deletion derivatives (i.e. pXK45, pXK47, andpXK50) were characterized for form I expression in three hostbackgrounds (i.e. E. coli, S. Typhi, and S. sonnei) (FIG. 1). Plasmidinserts ranging in size from 15.7 to 12.4 kb all directed form I antigenexpression in each host as shown by results of bacterial agglutinationof plasmid bearing subclones with form I-specific antiserum (FIG. 1B).However, this antiserum did not agglutinate bacteria containing pXK47,which contains an 11 kb insert, like the one previously reported (24) tocontain the entire form I biosynthetic region. In the present study, thesmallest inserts that directed form I antigen expression were the 12.7and 12.4 kb inserts of plasmids pXK50 and pXK46, respectively. However,form I specific agglutination of host strains containing pXK46 was weakand did not correlate with the detection of typical polymerizedO-antigen as described below.

TABLE 1 Bacterial strains and plasmids Strain/ PlasmidGenotype/Description Reference Strain E. Coli DH5α supE44, hsdR17,recA1, endA1, gyrA96, thi-1, (35) relA1 HB101 supE44, hsdS20, (r_(B)⁻m_(B) ⁻), recA13, ara-14, (35) proA2, lacY1, galk2, rpsL20, xyl-5,mtl-1 S. Typhi Ty21a galE, ilvD, viaB (Vi⁻), H₂S⁻ (14) S. sonnei 53GIForm I (phase I), virulent isolate (26) 53GII Form II (phase II),avirulent variant (26) pGB-2 pSC101 derivative, low copy plasmid;Sm^(r), Spc^(r)  (7) pBR325 pBR322-derived plasmid; Cm^(r), Ap^(r),Tc^(r)  (4) pUC18 pBR322-derived cloning vector; Lac⁺, Ap^(r) (41) pHC79pBR322-derived cosmid; Ap^(r), Tc^(r) (23) pCVD551 pHC79-derived cosmid;Cm^(r) Timothy McDaniel pWR101 S. sonnei form I positive cosmid clone#19; Tc^(r) (26) pWR102 S. sonnei form I positive cosmid clone #27;Tc^(r) (26) pXG914 30 kb BamHI fragment of pWR101 cloned into This studypCVD551 cosmid; Cm^(r) pXK68 15.7 kb HindIII fragment of pXG914 clonedinto This study pBR325; Ap^(r), Cm^(r) pXK67 15.7 kb HindIII fragment ofpXG914 cloned into This study pGB-2; Sm^(r), Spc^(r) pXK66 13.6 kbHindIII fragment from pXG914 cloned This study into pBR325; Ap^(r),Cm^(r) pXK65 13.6 kb HindIII fragment of pXK67 cloned into This studypGB-2; Sm^(r), Spc^(r) pXK45 13.3 kb HindIII-SmaI fragment of pXK45cloned This study into pGB-2; Sm^(r), Spc^(r) pXK50 12.7 kb HindIII-Pmelfragment of pXK45 cloned This study into pGB-2; Sm^(r), Spc^(r) pXK4612.4 kb HindIII fragment of pXK67 cloned into This study pUC18, Ap^(r)pXK47 11.0 kb′ XbaI-HindIII fragment of pXK46 cloned This study intopUC18; Ap^(r) pXK2.1 2.1 kb HindIII fragment of pXK67 cloned This studyinto pUC18; Ap^(r) pXK1.2 1.2 kb HindIII fragment of pXK67 cloned Thisstudy into pUC18; Ap^(r) pXK1.4 1.4 kb XbaI-HindIII fragment of pXK46cloned This study into pUC18; Ap^(r)

Plasmid-based expression of form I O-Ps in each host was furtherexamined by SDS-PAGE followed by silver-staining and Western blottingwith form I-specific antisera (FIG. 2). LPS from wildtype S. sonnei 53GIgave a typical O-antigen ladder pattern with the predominant chainlength of 20 to 25 O-units as detected by silver stain or immunoblotting(FIG. 2A). Immunoblotting also detected additional material of lowermobility, well above the position of 25 O-repeats, suggestingcapsule-like expression. As expected, anti-form I reactive material wasnot detected with S. sonnei 53GII or the “rough” E. coli HB101. However,recipient 53011 or HB101 carrying either pXK67 or pXK65 showed typicalLPS ladder patterns like that seen from parent strain 53GI. Similar LPSladder patterns were detected in further studies of E. coli carryingcosmid pWR 101, pXG914, pXK45 containing a 13.3 kb insert (FIG. 2B) orpXK50 containing a 12.7 kb insert (data not shown). In contrast, adramatic loss of form I immunoreactive material was noted in eitherShigella (FIG. 2A) or E. coli (FIG. 2B) containing pXK46, which has a12.4 kb insert. Moreover, host strains carrying pXK47, which has an 11kb insert, showed no reaction by silver staining or immunoblotting (datanot shown).

S. Typhi Ty21exhibited a typical and distinctive 9,12 LPS ladder patternby silver-stain analysis, but, as expected, showed no form I antigen byimmunoblotting (FIG. 2C). The presence of pXK67, pXK65, or pXK45 (FIG.2C) or the smaller pXK50 (data not shown) in Ty21a did not noticeablyaffect the silver-stained O-antigen pattern of this strain. However,immunoblot analysis revealed that these plasmids directed the expressionof anti-form I reactive material. The form I material in S. Typhi didnot migrate as LPS and presumably was attached to carrier lipid asproposed previously (37). No immunoreactive form I O-Ps was detected instrain Ty21a carrying pXK46. Thus, the combined results suggest thatplasmids pXK67, pXK65, pXK45, and pXK50, but not pXK46, contain theessential genes for synthesizing form I O-Ps in each of the three hoststrains examined.

Sequence Analysis of the Form I Gene Region.

A contiguous segment of about 18 kb was sequenced to characterize theform I biosynthetic gene region and evolutionarily important adjacentregions, (see FIG. IC; Genbank 1tAF294823). Primary analysis of thissequence revealed 18 ORFs, the properties of which are summarized inTable 2 and FIG. 1. The notably higher GC content for ORFS, ORFs 11through 13 and other terminal sequences, compared with the remainder ofthe form I region, suggests different evolutionary origins for thesesequences.

The inserts of all plasmids that direct the expression of typical form Iantigen (FIG. 1B) begin at the HindHI site located at nucleotideposition 1,310 of our sequence, in the middle of ORF3, a homolog of wzz.Ten identically oriented ORFs (i.e. ORFs 4 to 13) occur within the 12.7kb insert of pXK50, the smallest insert that directs typical form Iantigen expression. One of these ORFs (i.e. ORF 8 in FIG. 1C) representsthe transposase of IS630, which is inserted nonpolarly into theC-terminus of the preceding biosynthetic ORF as noted previously (38).All remaining ORFs present within the pXK50 insert are homologs of knowngenes for polysaccharide biosynthesis (Table 2), except ORFS, which wesuggest, encodes a C5-epimerase based on the need for such an enzyme inour proposed biosynthetic pathway (see Discussion). The presence of aputative promoter, identified by a −35 and −10 consensus sequence(ATTACCN₁₅TATAGT) (SEQ ID NO:12) at nucleotide positions 1,645 to 1,671of our sequence (i.e. AF294823, SEQ ID NO:7) and a typicaltranscriptional terminator, identified by a stem-loop structure andadjacent poly(T) sequence at nucleotide positions 13,930 to 13,949 ofSEQ ID NO:7 (and is SEQ ID NO:13) defines an essential 12.3 kb regionrequired for form I O-Ps biosynthesis by our plasmid subclones. Thisregion, which contains 10 intact ORFs, including the transposase ofIS630, begins near the 3′ end of ORF3 and ends between ORF13 and ORF14(FIG. 1C).

Sequencing of the operon-adjacent regions revealed several interestingfeatures that aid in understanding the evolution of the plasmid-borneform I region.

TABLE 2 Summary of S. sonnei 53G ORFs Gene Location in (G + C) aaIdentity Proposed function ORF Name Sequence % no. Selected Homolog(accession no.) % (aa^(a)) of 53G protein 1 insB 519-16  54.4 167 IS1(InsB), E. coli (AJ223474) 98 (167) IS1 transposase 2 insA 713-438 52.591 IS1 (InsA), E. coli (AJ223475) 100 (91)  IS1 protein 3 wzz  788-1,72036.4 310 Wzz, Actinobacillus actinomycetemcomitans (AB041266) 35 (328)chain length Wzz, E. coli (AF011911) 26 (292) determinant 4 wbgT1,756-3,069 36.1 437 WbpO, Pseudomonas aeruginosa (AF035937) 74 (418)UDP-GalNAc WcdA, Salmonella typhi (D14156) 63 (418) dehydrogenase 5 wbgU3,150-4,187 34.1 345 WbpP, Pseudomonas aeruginosa (AF035937) 67 (343)UDP-GlcNAc WcdB, Salmonella typhi (D14156) 65 (338) C4-epimerase 6 wzx4,276-5,556 28.1 426 Cps19CJ, Streptococcus pneumoniae (AF105116) 21(394) repeat unit Wzx, Escherichia coli (AF104912) 19 (393) transporter7 wzy 5,625-6,797 29.8 390 Cap14H, Streptococcus pneumoniae (X85787) 25(201) polysaccharide polymerase 8 IS630 6,894-7,925 52.8 343 IS630(ORF343), S. sonnei (P16943) 99 (343) IS630 transposase 9 wbgV7,958-9,202 29.9 414 None none UDP-GalNAcA C5-epimerase^(b) 10 wbgW 9,186-10,181 26.6 331 WaaV, E. coli (AF019746) 27 (237) glycosyltransfease LgtA, Neisseria gonorrhoeae (U14554) 30 (142) 11 wbgX10,178-11,332 37.6 384 WlbF, Bordetella bronchiseptica (AJ007747) 55(392) amino-sugar Per, E. coli (AF061251) 34 (383) synthetase RfbE,Vibrio cholerae (X59554) 31 (380) 12 wbgY 11,349-11,939 35.4 196 WlbG,Bordetella pertussis (X90711) 53 (194) glycosyl transferase WcaJ, E.coli K-12 (U38473) 34 (197) WbaP, E. coli K30 (AP104912) 31 (212) 13wbgZ 11,954-13,873 44.3 639 WbcP, Yersinia enterocolitica (Z47767) 68(633) UDP-GlcNAc WbpM, Pseudomonas aeruginosa (U50396) 49 (657)C6-dehydratase WlbL, Bordetella pertussis (X90711) 49 (592) C4-reductaseFlaA1, Helicobacter pylori (AE000595) 28 (297) 14 aqpZ 13,992-14,50455.5 170 ORF10P, P. shigelloides (AB025970) 99 (146) water channel AqpZ,E. coli (AE000189) 71 (146) protein 15 orfA 14,657-14,983 53.8 108 IS629(ORFA), S. sonnei (P16941) 99 (108) IS629 transposase 16 InsB16,706-15,486 55.0 406 IS91 (TnpA), E. coli (X17114) 94 (406) IS91transposase 17 InsA 17,071-16,706 53.0 121 IS91 (ORF121), E. coli(S23781) 95 (121) IS91 protein 18 InsB 17,130-17,978 54.8 282 IS911(InsB), S. dysenteriae (AAF28127) 99 (271) IS911 transposase ^(a)Lengthof comparable sequence in the homologous protein ^(b)Proposed functionbased on the predicted presence of an enzyme that converts UDP-GalNAcAto UDP-AltNAcA (see Discussion)Analysis of upstream sequences from pWRIOI subclones revealed thepresence of a partial wzz (933 bp) created by an IS1 insertion. Sequencehomology to the plasmid R100 was noted immediately 5′ of this IS1element (Xu et al., unpublished data; FIG. 3A). Unexpectedly, the 5′region of pWR 101 differed from that in pWR102. The latter plasmidcontained a partial IS91 (201 bp), a partial IS630 (339 bp), a JUMPstartsequence (i.e. CAGCGCTTTGGGAGCTGAAACTCAAGGGCGGTAGCGTA) (SEQ ID NO:14),which is characteristic of O-antigen loci and a full-length copy of wzz(1,104 bp) (FIG. 3A). The observation of a full length S. sonneiplasmid-borne wzz, as reported previously (38), preceded by a JUMPstartsequence and partial IS elements suggests that this pWR 102-derivedsequence represents that of the original 180 kb S. sonnei virulenceplasmid and that during subcloning of this region in pWR101, an ISIelement insertion occurred within wzz causing a 5′-deletion of this geneand adjacent upstream sequences (FIG. 1C). The remnants of IS630 andIS91 found upstream of JUMPstart in pWR102 suggests the insertion ofIS91 via its left inverted repeat (IRL) into a -GTTC- target site (33)originally present within IS 630 and subsequent deletion of much of theIS91 element (FIG. 3A).

Immediately downstream of the form I encoding region, a partial aqpZgene (513 bp) was found that is virtually identical to the 5′-portion ofPleisiomonas shigelloides aqpZ (699 bp) (6). Further downstream apartial 1S629 element (31), a small fragment of a Pseudomonas ISelement, a full-length IS91 and partial IS911 sequences were identified(FIG. 3B). The specific target sequence of IS91, -GTTC-, was foundimmediately adjacent to the right inverted repeat (IRR) of this element,indicating the prior insertion of IS91 into a target site originallypresent in the middle of IS911. Thus, the region downstream of the formI biosynthetic operon contains numerous IS element remnants, and likethe upstream region, serves as a recombinational hotspot.

Stability of Form I O-Ps Expression in a Salmonella Vaccine Vector.

Several recombinant plasmids were tested for their ability to directstable form O-Ps expression in S. Typhi Ty21a. Following electroporationof each plasmid into strain Ty21a, the resulting strain was grown in theabsence of antibiotic selective pressure for approximately 25 and 50generations and then examined for form I antigen expression. Thepercentage of form I-positive colonies was determined by immunoblotassay of colonies grown on LB agar without antibiotic. Salmonellaharboring the 15.7 kb form I region insert in the multicopy vectorpBR325 (i.e. pXK68) exhibited highly unstable form I O-Ps expression.Thus, following growth for 24 hrs, the loss of antigen expression fromSalmonella carrying this plasmid was greater than 97% (Table 3).Deletion of IS91 from the 15.7 kb insert of pXK68 to generate the 13.6kb fragment of pXK66 increased the stability of form I O-Ps expression.The percentage of form I positive colonies was further enhanced whenthese inserts were carried in the low copy vector, pGB2. The 15.7 kbinsert in pGB-2 pXK67) exhibited markedly improved stability of antigenexpression compared with the same insert in pBR325. Again, deletion of1S91 from the 15.7 kb insert of pXK67 to generate the 13.6 kb fragmentof pXK65 increased the stability of form I O-Ps expression. In fact, asshown in Table 3, pXK65 and pXK45 directed stable form I antigenexpression in Salmonella over 50 generations.

TABLE 3 Stability of plasmid-based form I O-Ps expression in S. TyphiTy21a^(a) Percent form I O-Ps positive colonies at: Plasmid VectorInsert (kb) 12 h 24 h pXK68 pBR325 15.7 12.5 2.5 pXK66 PBR325 13.6 8045.5 pXK67 pGB-2 15.7 78 69 pXK65 pGB-2 13.6 100 98.5 pXK45 pGB-2 13.3100 97 ^(a)A form I positive colony of each strain was inoculated inL-broth and grown for 12 h (approximately 25 generations) beforedilution and regrowth in fresh L-broth for an additional 12 h. Samplestaken at 12 or 24 h were plated on L-agar and the resulting coloniesassayed for form I O-Ps by colony immunoblotting.

Vaccine Protection Study in Mice.

Shigellae are specific for higher primates and nonprimate models do notexist for the development of either typical dysenteric disease from lowinfectious doses of these bacteria or protective immunity from naturalchallenge. Nevertheless, mice have been employed previously todemonstrate immune stimulation by a vaccine and specific protectionagainst parenteral challenge with virulent S. sonnei (12). In thepresent study, ICR mice were immunized with a single ip dose of viableS. Typhi Ty21a containing pXK65 or pXK45, Ty21a alone, or saline andchallenged at 5 weeks post-immunization with 5×10⁵ virulent S. sonnei53GI (i.e. approximately 100xLD₅₀). This challenge resulted in 100%mortality in negative control mice immunized with saline or strain Ty21aalone (Table 4). In contrast, all mice that received Ty21a harboring thestable form I inserts deleted for IS91 and carried by pGB-2 wereprotected from the S. sonnei challenge.

TABLE 4 Mouse protection against virulent S. sonnei challenge Vaccine(plasmid)/control^(a) Suvivors/total^(b) S. Typhi Ty21a (pXK45) 8/8 S.Typhi Ty21a (pXK65) 8/8 S. Typhi Ty21a 0/8 Saline 0/8 ^(a)Vaccinestrains containing plasmids or control Ty21a alone were suspended insaline to a concentration of 2.5 × 10⁷ cells per 0.5 ml dose forintraperitoneal immunization. Saline (0.5 ml) served as control.^(b)Each mouse was challenged intraperitoneally with 5 × 10⁵ CFU S.sonnei 53GI (i.e. 100 × LD₅₀) in 0.5 ml saline containing 5% hog gastricmucin and monitored for four days.

III. Discussion

The genes controlling form I O-Ps biosynthesis have previously beencloned and sequenced to varying extents as summarized in FIG. 4 (D. J.Kopecko, L. S. Baron, T. L. Hale, S. B. Formal and K. Noon, Abstr. 83thAnnual Meeting of the American Society for Microbiology, abstr. D 10,1983) (6, 24, 38, 42, 45). However, reported sequence differences in theS. sonnei form I gene region (FIGS. 4 A and B), combined with limitedanalyses of LPS expression, have resulted in confusion regarding theessential genes for form I antigen biosynthesis. Houng and Venkatesan(24) reported that these genes were contained within an 11 kb region ofthe S. sonnei 53GI virulence plasmid; DNA sequencing revealed ten ORFsincluding IS630 (FIG. 4B). However, our findings, which support otherrecent sequencing studies of the form I gene region in S. sonnei strains53GI and HW383 (FIGS. 4A and C), as well as the corresponding generegion of P. shigelloides (FIG. 4D), suggest that the form Ibiosynthetic region contains an additional gene, designated wbgZ (FIG.4A), homologs of which occur in many Ps gene clusters (5) but not in thesequence of Houng and Venkatesan (24) (FIG. 4B).

Antibody to form I O-Ps was previously reported to agglutinate subclonesexpressing an 11 kb form I insert (24), which lacks wbgZ. In contrast,we found that such subclones (i.e. pXK47) were not agglutinated byspecific anti-form I antibody, prepared by absorption with form II S.sonnei cells. Further, LPS analysis by silver stain or immunoblot showedno detectable form I material from subclones expressing the 11 kbinsert, but typical form I LPS from pXK50 subclones expressing the 12.7kb insert thereby indicating that wbgZ (but not aqpZ) is required forform I O-Ps biosynthesis. The right-hand end of the form I gene region,between wbgZ and aqpZ, is further defined by the presence of atranscriptional terminator in this region and the dramatic effect onform I O-Ps synthesis seen from the short truncation of wbgZ insubclones expressing the 12.4 kb insert (FIG. 2, pXK46).

The left-hand end of the essential form I region is defined by plasmidinserts that begin in the middle of wzz (FIG. 1B) but direct thesynthesis of typical form I LPS. The wild type distribution of LPS chainlength seen in our S. sonnei subclones (FIG. 2A) can be explained by theexpression of the previously described chromosomal wzz (38), whichapparently determines the chain length of form I LPS. Whereas JUMPstart,a presumed transcriptional antiterminator (43), and plasmid borne wzzmay play a role in biosynthesis of LPS by wild type S. sonnei and P.shigelloides 017, our studies indicate that neither of these loci isessential for form I O-Ps expression from our subclones. Suchobservations also suggest the presence of a promoter at the 3′ end ofplasmid borne wzz (6), immediately ahead of wbgT, the first essentialgene for plasmid-based form I O-Ps biosynthesis. The IS630 elementinserted in the C-terminus of ORF7 (nucleotides 6894-7925 of SEQ ID NO:7which is SEQ ID NO:15) (i.e. wzy) (38) is evidently also not essentialfor form I O-Ps biosynthesis as the comparable region of P.shigelloides, which lacks IS630, also directs the production of typicalLPS. Thus, the available data from studies of LPS biosynthesis clearlyindicate that nine genes beginning with wbgT (ORF4) and ending with wbgZ(ORF13) (FIG. 4A) are required for form I antigen biosynthesis in eachof the three host genera examined.

The properties of these nine essential genes (Table 2) provide the basisfor the detailed biosynthetic pathway presented as a working hypothesisin FIG. 5. These genes include two (i.e. wbgW and wbgY) for putativeglycosyl transferases and two (i.e. wzx and wzy) for proteins thatfunction in the transport and polymerization of form I repeating units.Thus, the remaining five genes of the form I cluster may function toconvert available nucleotide-linked sugars to the 4n-D-FucNAc- andL-AItNAcA-containing precursors of the form I disaccharide repeatingunit (25). The initial step in formation of UDP-4-n-D-FucNAc waspreviously proposed to involve conversion of UDP-GlcNAc toUDP-4-keto-6-deoxy-GlcNAc by the action of wbgV (38). Rather than wbgV,we suspect that wbgZ catalyzes this reaction. Homologs of wbgZ, whichinclude FlaA1 of Helicobacter pylori and WbpM of Pseudomonas aeruginosa,are associated with synthesis of the 2,6-deoxysugars QuiNAc, O-FucNAc,and structurally related derivatives such as 4-n-D-QuiNAc (5), theC4-epimer of 4-n-D-FucNAc. Significantly, FlaA 1 of H. pylori hasrecently been identified as a bifunctional UDP-GlcNAcC6-dehydratase/C4-reductase that catalyzes the conversion of UDP-GlcNAc25, to UDP-QuiNAc through the stable intermediate,UDP-4-keto-6-deoxy-GlcNAc (8). Consequently, the predicted intermediateproduct of wbgZ, UDP-4-keto-6-deoxy-G1cNAc, is the putative substrate ofwbgX (38), which likely catalyzes the formation of 4-n-D-FucNAc (FIG. 5)in a manner similar to the conversion of GDP-4-keto-6-deoxymannose toGDP-perosamine by perosamine synthase of V. cholerae O1 (39) and E. coli(2).

Homologs of two other S. sonnei biosynthetic genes, wbgT and wbgU, occurin a number of bacteria that synthesize N-acetylgalactosamine uronicacid (GaINAcA) including P. aeniginosa serotype 06 (1) andVi-capsule-expressing Salmonella serovars (19) (Table 2). The relevantbiosynthetic pathway, proposed from studies of P. aeruginosa (1),involves the conversion of UDP-GlcNAc to UDP-GaINAc by WbpO andsubsequent conversion of UDP-GaINAc to UDP-N-GaINAcA by WbpP. Indeed,recent biochemical studies confirm the identification of WbpP as aUDP-GlcNAc C4-epimerase (8) and WbpO as a UDP-GaINAc dehydrogenase (46).Significantly, D-GaINAcA, the predicted product of WbgT, is theC5-epimer of L-AItNAcA, a constituent of form I O-Ps. Thus, thecorresponding precursor, UDP-L-AItNAcA, would be obtained by the actionof a C5-epimerase on UDP-GaINAcA. We predict that this activity isprovided by WbgV (FIG. 5), the only S. sonnei ORF that failed toretrieve significant homologs from the database (Table 2). Although weakhomology between WbgV and plant NADH dehydrogenases was previouslyreported (38), we found that WbgV is not affiliated with these or otherNADH-containing enzymes in the Blocks Data Base (Fred Hutchinson CancerResearch Center) thereby questioning the identification of WbgV as adehydrogenase. Intracellular C5-epimerases that act on nucleotide-linkedsugars have not been described to our knowledge, which may contribute tothe apparent absence of WbgV homologs in the database. ExtracellularC5-epimerases that act on polysaccharides are, however, well documentedand include the enzymes of P. aeruginosa (13) and Azotobacter vinelandii(11) that convert D-mannuronic acid to L-guluronic acid in alginatepolymers as well as mammalian enzymes that convert D-glucuronic acid toL-iduronic acid in heparin and heparin sulfate (30).

That the form I O-Ps is linked to the phase II core of S. sonnei (25)through 4-n-D-FucNAc suggests that 4-n-D-FucNAc is the first sugarattached to the acyl carrier lipid. This step almost certainly dependson WbgY, which is a homologue of several well-studied glycosyltransferases that link the first sugar of different O-antigen repeatingunits to carrier lipid (Table 2). WbgW, the other predicted glycosyltransferase (Table 2) presumably completes the biosynthetic unit bytransferring L-AltNAcA thereby forming L-AltNAcAα(1→3)4-n-D-FucNAc-PP-und. Indeed, the predicted α(1→3) transferof L-AItNAcAα by WbgW would resemble the known 13(143) transfer ofD-sugars by WaaV (20) of E. coli and LgtA of N. gonorrhoeae (16) (Table2). Wzx, a member of the PST(2) subfamily of polysaccharide transportproteins (34), based on its predicted size (Table 2) and hydropathyprofile (results not shown), would then be expected to flip thelipid-linked repeating unit from the cytoplasmic to periplasmic face ofthe plasma membrane without the aid of auxiliary export proteins.Wzx-mediated transport would provide the substrate for Wzy-dependentpolymerization resulting in the formation of a β1-4 linkage between eachadjacent repeating unit, thereby completing the form I O-Ps structure(FIG. 5).

Plasmid-based expression of form I O-Ps in S. typhi Ty21a, which has acore that is chemically dissimilar to that of Shigellae, resulted in theproduction of a lipid-linked surface Ps (37) rather than typical form ILPS (FIG. 2C). In contrast, a significant fraction of form I O-Pssynthesized in S. sonnei and E. coli was ligated to core-Lipid A.However, even from these species, a slow migrating band of form Iimmunoreactive material, apparently unlinked to core-Lipid A, wasdetected (FIGS. 2A and B). It is unclear whether this band ofcore-nonlinked form I material is surface bound through the acyl carrierlipid, or alternatively through another molecule as an O-antigencapsule. As pointed out in a recent review (44), O-Ps capsules areeasily overlooked because serological and structural studies havegenerally been interpreted with the expectation that all surface Oantigen is core-lipid A linked. However, examples such as E. coilserotype O111 have long been recognized (15) in which the same O-Ps issurface expressed in a LPS form and in an LPS-unlinked capsular form.Clearly, further studies of S. sonnei form I O-Ps are needed to clarifythis possibility.

High homology between the gene regions for O-Ps biosynthesis in S.sonnei and P. shigelloides (6, 38), over the region from wzz to aqpZ(FIG. 4), supports the proposal of Reeves and coworkers (29) that S.sonnei evolved from E. coli by the acquisition of the form Ibiosynthetic region from P. shigelloides. The form I operon adjacentsequences obtained herein (FIGS. 1B and 3) provide an improveddefinition of the limits of the gene transfer event. Comparison of theavailable S. sonnei form I gene region sequences (FIG. 4A) with theanalogous Pleisiomonas region (FIG. 4D) suggests the transfer ofapproximately 12.6 kb of P. shigelloides chromosomal DNA. The right-handendpoint apparently occurred at by 513 within aqpZ where sequencehomology between P. shigelloides and S. sonnei ends abruptly. Theleft-hand junction apparently occurred upstream of JUMPstart wherepartial IS elements were identified in pWR102 (FIG. 3). Since remnantsof IS91, IS630, and other elements have been shown to flank the form 1operon in S. sonnei (FIGS. 3 and 4A), any of these elements could havebeen involved in transposition of this region, likely from thePleisiomonas chromosome to a plasmid,'which was then transferred to theevolving E. coli recipient.

Form I antigen expression is frequently lost in S. sonnei mainly byspontaneous loss of the large virulence plasmid (26). Instead ofstabilizing form I expression in attenuated Shigella for use as a livevaccine, our approach has been to transfer the form I genes into S.Typhi Ty21a. Ty21a (14) is a proven safe and effective,mucosally-delivered, live bacterial vaccine which stimulates long-termprotection against typhoid fever. In addition, Ty21a has the advantageof oral administration, eliminating the need for needles, syringes and askilled health professional for immunization. A live, oral candidatevaccine strain, 5076-IC, was previously constructed by introducing thelarge S. sonnei virulence plasmid into Ty21a. The resulting strain wasprotective in humans challenged with virulent S. sonnei (3, 12, 21) butwas genetically unstable, resulting in loss of form I O-Ps expression(17). The current study has allowed us to create stable, minimal-sizedS. sonnei form I region constructs in Ty21a. The stability ofplasmid-based expression of form I O-Ps was enhanced by deletion of thedownstream IS91 from form I inserts and was further stabilized by use ofthe low copy vector pGB-2 (Table 3). Animal studies (Table 4) haveprovided preclinical evidence that these minimal-sized form I regionconstructs in S. Typhi induced protective immunity in a stringent mousechallenge model.

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It is understood that the examples and embodiments described herein arefor illustrative purposes only and that various modifications or changesin light thereof will be suggested to persons skilled in the art and areto be included within the spirit and purview of this application andscope of the appended claims. All publications, patents, and patentapplications cited herein are hereby incorporated by reference in theirentirety for all purposes.

1. An immunoprotective composition comprising an attenuated bacteriaexpressing an antigen comprising the S. sonnei form I O-polysaccharide,wherein the attenuated bacteria comprises an expression cassettecomprising the genes wbgT, wbgU, wzx, wzy, wbgV, wbgW, wbgX, wbgY, andwbgZ isolated from the S. sonnei db/rIb gene cluster or Plesiomonasshigelloides (P. shigelloides) O17 gene cluster operably linked totranscriptional promoter and termination signals, and wherein theexpression cassette does not include at least one of the followinggenes: IS1, wzz, IS630, aqpZ, IS629, IS91 , and IS911.
 2. Theimmunoprotective composition of claim 1, wherein said attenuatedbacteria are selected from the group consisting of Campylobacter jejuni,Campylobacter coli, Listeria monocytogenes, Yersinia enterocolitica,Yersinia pestis, Yersinia pseudotuberculosis. Escherichia coli, Shigellaflexneri, Shigella sonnei, Shigella dysenteriae, Shigella boydii,Helicobacter pylori, Helicobacter fells, Gastrospirillum hominus, Vibriocholerae, Vibrio parahaemolyticus, Vibrio vulnificus, Bacteroidesfragilis, Clostridium difficile, Salmonella typhimurium, Salmonellatyphi, Salmonella gallinarum, Salmonella pullorum, Sahnonellacholeraesuis, Salmonella enteritidis, Streptococcus gordonii,Lactobacillis sp., Klebsiella pneumoniae, Enterobacter cloacae, andEnterococcus faecalis.
 3. The immunoprotective composition of claim 2,wherein said Escherichia coli bacteria is the strain DH5α.
 4. Theimmunoprotective composition of claim 2, wherein said Escherichia colibacteria is the strain HB101.
 5. The immunoprotective composition ofclaim 2, wherein said V. cholera bacteria comprises a cixA genemutation.
 6. The immunoprotective composition of claim 2, wherein saidSalmonella typhi bacteria are selected from the group consisting of thestrains CVD 908, CVD 908-hirA, X4073, and Ty800.
 7. The immunoprotectivecomposition of claim 2, wherein said Shigella sonnei bacteria areselected from the group consisting of strains 53GI and 53GII.
 8. Theimmunoprotective composition of claim 1, wherein said fragment comprisesSEQ ID NO: 2, SEQ ID NO: 3, or SEQ ID NO: 4 operably linked to apromoter.
 9. The immunoprotective composition of claim 1, wherein saidfragment lacks SEQ ID NO:
 15. 10. The immunoprotective composition ofclaim 1, wherein said antigen is expressed from a recombinant plasmid.11. The immunoprotective composition of claim 10, wherein saidrecombinant plasmid contains a selectable marker.
 12. Theimmunoprotective composition of claim 11, wherein said selectable markeris aspartate β-semialdehyde dehydrogenase (asd) gene operably linked toa promoter.
 13. The immunoprotective composition of claim 10, whereinsaid recombinant plasmid comprises SEQ ID NO: 2, SEQ ID NO: 3, or SEQ IDNO: 4 operably linked to a promoter.
 14. The immunoprotectivecomposition of claim 10, wherein said recombinant plasmid comprises SEQID NO: 2 operably linked to a promoter.
 15. The immunoprotectivecomposition of claim 10, wherein said recombinant plasmid comprises SEQID NO: 3 operably linked to a promoter.
 16. The immunoprotectivecomposition of claim 10, wherein said recombinant plasmid comprises SEQID NO: 4 operably linked to a promoter.
 17. The immunoprotectivecomposition of claim 13, wherein said recombinant plasmid lacks SEQ IDNO:
 15. 18. The immunoprotective composition of claim 1 furthercomprising a pharmaceutical diluent.
 19. A plasmid comprising apolynucleotide comprising the genes wbgT, wbgU, wzx, wzy, wbgrf, wbgW,wbgX, wbgY, and wbgZ from the S. sonnei rfb/rfc gene cluster orPlesiomonas shigelloides (P. shigelloides) O17 gene cluster operablylinked to transcriptional promoter and termination signals, wherein thepolynucleotide does not include at least one of the following genes:IS1, wzz, IS630, aqpZ, IS629, IS91 and IS911.
 20. The plasmid of claim19, wherein the plasmid encodes an antigen comprising the S. sonnei formI O-polysaccharide.